The structures, distributions, and functions of molecules of importance in the nervous system are being studied. Several cDNAs that encode neurotransmitter receptors--among these, receptors for vasopressin, oxytocin, and adenosine--have been isolated. In addition, cDNAs encoding serotonin and dopamine transporters have been cloned. The distributions of mRNAs to which the above DNAs are complementary have been or are being determined by means of in situ hybridization histochemistry (ISHH) as have the distributions of other mRNAs (e.g., peptide processing proteases, components of second messenger systems). Expression of cloned cDNAs has allowed functional studies to be undertaken.